A capacitively coupled plasma processing apparatus includes an upper electrode and a lower electrode arranged in parallel to each other within a processing vessel. A processing target substrate (e.g., a semiconductor wafer, a glass substrate, etc) is mounted on the lower electrode, and a high frequency power having a frequency (typically, about 13.56 MHz or higher) suitable for plasma generation is applied to the upper electrode or the lower electrode. Electrons are accelerated by a high frequency field generated between the two facing electrodes by applying the high frequency power, and plasma is generated as a result of ionization by collision between the electrons and a processing gas. Through a gas phase reaction or a surface reaction of radicals or ions included in the plasma, a thin film is formed on the substrate, or a material or a thin film on a surface of the substrate is etched.
Recently, as a design rule is getting more miniaturized in a manufacturing process of a semiconductor device or the like, higher level of dimensional accuracy is required in, especially, plasma etching. Further, it is required to increase etching selectivity against a mask or an underlying film and to improve etching uniformity in the entire surface of a substrate. For this reason, pressure and ion energy in a processing region within a chamber tends to be reduced, and a high frequency power having a high frequency equal to or higher than about 40 MHz is used.
However, as the pressure and the ion energy are reduced, an influence of a charging damage, which has been negligible conventionally, can be no more neglected. That is, in a conventional plasma processing apparatus having high ion energy, no serious problem may occur even when a plasma potential is non-uniform in the entire surface of the substrate. However, if the ion energy is lowered at a lower pressure, the non-uniformity of the plasma potential in the entire surface of the substrate may easily cause the charging damage on a gate oxide film.
To solve this problem, using a power modulation process of modulating a high frequency power for plasma generation in an on/off (or H level/L level) pulse shape is considered to be effective (Patent Document 1). According to this power modulation process, a plasma generation state in which plasma of a processing gas is being generated and a plasma non-generation state in which plasma is not being generated are alternately repeated at a preset cycle during a plasma etching process. Accordingly, as compared to a typical plasma process in which plasma is continuously generated from the beginning of the process to the end thereof, a time period during which plasma is continuously generated may be shortened. Accordingly, the amount of electric charges introduced into a processing target substrate from the plasma at one time or the amount of electric charges accumulated on the surface of the processing target substrate may be reduced, so that the charging damage is suppressed from being generated. Therefore, a stable plasma process can be performed and reliability of the plasma process can be improved.
Further, in the capacitively coupled plasma processing apparatus, a RF bias method is widely employed. In this RF bias method, a high frequency power having a relatively low frequency (typically, about 13.56 MHz or lower) is applied to the lower electrode on which the substrate is mounted, and ions in the plasma are accelerated and attracted to the substrate by a negative bias voltage or a sheath voltage generated on the lower electrode. In this way, by accelerating the ions in the plasma and bringing them into collision with the surface of the substrate, a surface reaction, anisotropic etching or modification of a film may be facilitated.
However, when performing the etching process to form via holes or contact holes by using the capacitively coupled plasma etching apparatus, a so-called micro-loading effect may occur. That is, an etching rate may differ depending on the hole size, so that it is difficult to control an etching depth. Especially, the etching rate tends to be higher at a large area such as a guide ring (GR), whereas the etching rate tends to be lower at a small via in which CF-based radicals are difficult to be introduced.
To solve this problem, a power modulation process of modulating a high frequency power for ion attraction in a first level/second level (or on/off) pulse shape and varying the duty ratio thereof is deemed to be effective (see, for example, Patent Document 2). According to this power modulation process, a period of maintaining a high power of the first level (H level) suitable for etching a preset film on the processing target substrate and a period of maintaining a low power of the second level (L level) as a high frequency power for ion attraction suitable for depositing polymer on a preset film on the processing target substrate are alternately repeated at a certain cycle. Accordingly, at an area having a larger hole size, a proper polymer layer may be deposited on the preset film at a higher deposition rate, so that the etching may be suppressed. Thus, an undesirable micro-loading effect may be reduced, and it may be possible to perform an etching process with a high selectivity and a high etching rate.
Further, in the capacitively coupled plasma etching apparatus, an organic mask having a low etching resistance, such as ArF photoresist, may be modified by applying a negative DC voltage to the upper electrode facing the substrate with a plasma generation space therebetween and attracting secondary electrons generated in the upper electrode into a surface layer of the substrate at a high speed. Recently, in order to improve the effect of modifying the organic mask by the high-speed electrons, there has been proposed a method of turning on and off a high frequency power for plasma generation with a regular pulse frequency and, synchronously, applying a DC voltage only during a period when the high frequency power is off (see, for example, Patent Document 3). As in this method, by applying the DC voltage to the upper electrode during a period when the high frequency power is turned off and, thus, a plasma sheath is thinned, the secondary electrons from the upper electrode may reach the substrate efficiently, so that the organic film on the substrate can be enhanced.